The role of surface antigens of lymphoid cells in determining inherited differences in the susceptibility of chickens to oncogenic viruses will be investigated as follows: The alloantigens (individually inherited antigens) on cells from the bursa of Fabricius and thymus, and derived peripheral lymphocytes (B- or T-cells), which we have already identified and established as determined by two independent autosomal loci Bu-1 and Th-1, will be tested for their relation to viral oncogenesis. Specifically, both B- and T-cell alloantigens will be tested for association with inherited differences in susceptibility to infection by lymphoid leukosis and sarcoma viruses, subsequent oncogenesis, and tumor regression. Similar tests will be made for Marek's disease herpesvirus. Both sets of antigens will also be studied for association with specific cellular and humoral immune responsiveness, and with lymphoproliferative potential, in extension of our preliminary findings. The mechanism of inherited resistance to lymphoid leukosis will be studied as follows: Bursa-inhibited chicks of one leukosis virus susceptibility genotype will be repopulated with bursal cells from chickens of a contrasting susceptibility genotype and studied for pathological changes following virus inoculation. The possibility will be tested of utilizing antialloantigen sera to suppress the development, in heterozygous chicks, of those bursal cells and derived B-cells which are susceptible to lymphoid leukosis virus.